Photochromic plastic materials, particularly plastic materials for optical applications, have in recent years attracted growing commercial interest due to their light weight in comparison to glass. Generally, these optical plastics incorporate organic photochromic compounds. When exposed to light radiation containing ultraviolet (UV) rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp, these photochromic compounds exhibit a reversible change in color. When the ultraviolet radiation is discontinued, the photochromic compound returns to its original color or colorless state. Ideal photochromic compounds for use in plastic optical applications change color efficiently upon exposure to near ultraviolet light, resist bleaching in white light and have a relatively fast fade rate.
A variety of organic photochromic compounds are already known in the art. U.S. Pat. No. 3,567,605 to Becker describes a series of pyran derivatives, including certain benzopyrans and naphthopyrans. U.S. Pat. No. 4,563,458 describes chroman and chromene compounds intended for use in pharmaceutical compositions. European Patent Publication 246, 114 and U.S. Pat. No. 4,826,977 describe a series of photochromic spiropyrans in which an adjacent group is attached at the position adjacent to the oxygen in the pyran ring. U.S. Pat. No. 4,818,096 and European Patent Publication 250,193 describe photoreactive plastic lenses that are coated or impregnated with the photochromic spiropyrans of European Patent Publication 246,114 in combination with a blue photochromic benzopyran or naphthopyran having an aminophenyl substituent at the position adjacent to the oxygen in the pyran ring. European Patent Publication 294,056 describes a process for producing a polyurethane plastic having photochromic properties wherein the photochromic compounds described include, among others, a naphthopyran derivative in which the pyran ring is substituted at the position adjacent to the oxygen in the pyran ring with di(p-methoxyphenyl) substituents.
Padwa et al. in J. Org. Chem., Volume 40, No. 8, 1975, page 142, describes the investigation of photochemical reactions of 2,2-dimethylbenzopyran and related compounds, identifies fatigue products and suggests pathways to the ring-opened colored intermediates and the phenolic degradation products.
Notwithstanding the existence of a number of known organic photochromic compounds, the extent of the photochromicity characteristic among various organic compounds has yet to be completely investigated. Further identification of new organic photochromic compounds is of value to the ophthalmic lens and related industries because, for example, different photochromic compounds can have vastly different physical properties other than photochromicity and therefore may well have particular advantages in certain applications. A need therefore remains to identify novel compounds which, among their other properties, are photochromic.